Bulletin of the American Physical Society
17th Biennial International Conference of the APS Topical Group on Shock Compression of Condensed Matter
Volume 56, Number 6
Sunday–Friday, June 26–July 1 2011; Chicago, Illinois
Session L5: Spectroscopy and Optical Studies III |
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Chair: Matt McCluskey, Washington State University Room: Renaissance Ballroom D |
Wednesday, June 29, 2011 9:15AM - 9:30AM |
L5.00001: Computation of Raman Spectra from Density Matrix Linear Response Theory in Extended Lagrangian Born-Oppenheimer Molecular Dynamics Anders Niklasson, Joshua Coe, Marc Cawkwell Linear response calculations based on density matrix perturbation theory [A. M. N. Niklasson and M. Challacombe, Phys. Rev. Lett. \textbf{92}, 193001 (2004)] have been developed within a self-consistent tight-binding method for extended Lagrangian Born-Oppenheimer molecular dynamics [A. M. N. Niklasson, Phys. Rev. Lett., \textbf{100}, 123004 (2008)]. Besides the nuclear coordinates, extended auxiliary electronic degrees of freedom are added to the regular Born-Oppenheimer Lagrangian, both for the electronic ground state and response densities. This formalism enables highly efficient, on-the-fly, analytic computations of the polarizability autocorrelation functions and the Raman spectra during energy conserving Born-Oppenheimer molecular dynamics trajectories. We will illustrate these capabilities via time-resolved Raman spectra computed during explicit, reactive molecular dynamics simulations of the shock compression of methane, benzene, tert-butylacetylene. Comparisons will be made with experimental results where possible. [Preview Abstract] |
Wednesday, June 29, 2011 9:30AM - 9:45AM |
L5.00002: Coherent Raman spectroscopy of laser driven shocks Cynthia Bolme, Shawn McGrane, Nhan Dang, David Moore We have previously performed infrared absorption and broadband visible absorption spectroscopy during shock loading of chemically reactive materials, including thin films of polyvinyl nitrate. While absorption spectroscopy has enabled us to infer the kinetics of chemical initiation, we desire a more conclusive diagnostic of the mechanisms and species involved. Towards this aim, we have been investigating various types of coherent Raman spectroscopies for use on ultrafast laser driven shocked materials. Recent results will be presented. [Preview Abstract] |
Wednesday, June 29, 2011 9:45AM - 10:00AM |
L5.00003: Raman spectroscopy study of laser-shocked TATB-based explosives Philippe Hebert, Viviane Bouyer, Joel Rideau, Michel Doucet, Louis-Pierre Terzulli We have developed a single-pulse Raman spectroscopy experiment dedicated to the study of laser-shocked explosives. The shocks were generated using the laser-driven flyer technique. The nanosecond pulse of a Nd:YAG laser ($\lambda $ = 1064 nm) was focused on a BK7 substrate coated with an ablation and an aluminium (Al) layer. The Al layer was used as the flyer. Our set-up can provide flyer velocities in the range 1500 to 3000 m/s with 15 $\mu $m thick Al foils which yields to maximum peak pressure of about 20 GPa on TATB samples. Single-shot Raman spectroscopy of shocked samples was performed using the second harmonic of a nanosecond Nd:YAG laser ($\lambda $ = 532 nm) focused on the back side of the samples through a glass window. In order to estimate the pressure of the probed sample, measurements of the particle velocity at the explosive-window interface were performed at the same time using the Photon Doppler Velocimetry system developed at CEA. Hydrodynamic simulations were also carried out. The first results obtained with this set-up on pressed samples of TATB based explosives are presented in this paper. [Preview Abstract] |
Wednesday, June 29, 2011 10:00AM - 10:15AM |
L5.00004: Laser-driven shock waves in a thin liquid layer David Veysset, Thomas Pezeril, Gagan Saini, Steve Kooi, Keith Nelson Optical shock generation and imaging techniques have been developed to allow direct real-time visualization of a converging shock front in a few micron thick liquid layer. The optical set-up includes an axicon that focuses an intense picosecond excitation pulse into a ring-shaped pattern in a water layer. Optical excitation induces a shock wave that propagates in the plane of the sample and converges toward the center resulting in cylindrical focusing of the shock front. Streak-camera images with a quasi-cw probe beam yielded real-time continuous time-resolved images of the entire shock propagation. Talbot imaging and interferometry with a femtosecond probe pulse were used to obtain full field images at variable delays. Shock pressure values calculated from the velocity of the shock front were found in agreement with refractive index changes determined from interferometric images, demonstrating the effect of shock focusing. The configuration of the experiment provides ample access for optical diagnostics of the shocked material and can be combined with a wide range of spectroscopic probes. [Preview Abstract] |
Wednesday, June 29, 2011 10:15AM - 10:30AM |
L5.00005: Towards initiation of explosives utilizing ultrafast laser quantum control Margo Greenfield, Shawn McGrane, Jason Scharff, David Moore Time dependent shaped electric fields can be utilized to control energetic materials by driving their reaction to initiation. This direct quantum controlled initiation (QCI) employs ultrafast shaped ultraviolet light to optimally control the explosives initiation reaction. QCI will enhance the understanding of energetic material reactions by yielding insight into the characteristics, such as reaction dynamics, necessary for initiation. Initial investigation into solutions of hexanitroazobenzene (HNAB), trinitroanaline (TNA), 1,1-diamino-2,2-dinitroethene (FOX-7), and diaminoazozyfurazan (DAAF) have been performed. Novel transient absorption spectra have been obtained for each material and note worthy regions have been further investigated for simple control response. The explosives not controlled through a single parameter have been further investigated with complex control. Further experimentation will be performed to explore the effect of QCI on thin films as the optimally shaped ultrafast laser pulses guide the energy flow along the desired paths. [Preview Abstract] |
Wednesday, June 29, 2011 10:30AM - 10:45AM |
L5.00006: Crystallization behavior of vapor-deposited hexanitroazobenzene (HNAB) films Robert Knepper, Alexander Tappan, Kathy Alam, Mark Rodriguez Hexanitroazobenzene is an interesting material for microenergetic research on explosive behavior at sub-millimeter geometries due to its small critical thickness for detonation and its chemical stability at temperatures above its melting point, which allows for fast deposition rates. HNAB films have been observed to deposit in an amorphous state, provided the substrate remains sufficiently cool during deposition. These amorphous films crystallize over a period of hours to weeks, depending on the ambient temperature, to a structure consisting of primarily HNAB-II crystallites. Several films were deposited to a thickness of $\sim $ 100 microns and subjected to a variety of temperatures ranging from 30 -- 75$^{\circ}$C to observe crystallization behavior. Crystallization rates were observed using time-lapse optical microscopy and were also characterized using scanning electron microscopy, atomic force microscopy, x-ray diffraction, and Raman spectroscopy at various stages of crystallization. [Preview Abstract] |
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